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Old 01-09-2015, 04:45   #16
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Re: Glueing plywood to fibreglass.

Battens. It gives the most flexibility. On Vigah the battens are horizontal and where there was a lot of curvature we used two layers of 6mm ply. The under layer marine pine and the outer teak in our case followed by re-attaching the decorative slats.

As far as insulating the hull, I decided not to as areas of the overhead we tried it on just encouraged mold in the insulation. Just the air gap between the hull and the interior gives a lot of insulating capacity and does allow more air movement next the the heating / cooling of the hull.
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Old 01-09-2015, 04:54   #17
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Re: Glueing plywood to fibreglass.

Expanding polyutherane, easy to use, quick curing, 100% waterproof, strong, easy to clean off excess with wood working tools.

Does require some clamping or though fixing.

I perfer Purbond.
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Old 01-09-2015, 08:30   #18
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Re: Glueing plywood to fibreglass.

Liquid nails will work O.K as long as it's not used as a filler. If the surfaces fit flush together it will stay stuck forever but it won't work if used to fill gaps. The main difference between epoxy and polyester resin is that epoxy is a glue. If you decide to go with epoxy, thicken it to a buttery paste using micro balloons from a boat-yard. It's a fifth of the price compared to the West System fillers.

My recommendation would be to remove the old resin. Of course, that's easier said than done but it is just as much work to cut the ply to fit and then gluing it. Use a hammer and chisel to remove the worst of it, then get one of these on your angle grinder and sand it off. Using anything smaller will take you weeks. It's just about the most horrible job to do with boats but at the end you will have a surface smooth enough onto which you can glue vinyl or whatever covering you choose.


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Old 01-09-2015, 09:31   #19
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Re: Glueing plywood to fibreglass.

Quote:
Originally Posted by Reefmagnet View Post
Well, threw the old boat up on the hard for a couple of months to do a few jobs which has expanded to include replacing all the tired plywood lining the inside of the hull and the bulkheads inside the saloon (thanks, Dear!). The original plywood was glued 30 odd years ago with thickened polyester resin on the hull which was a bit hit and miss but is extremely tenacious where it was a "hit" (probably because the hull was still "green" when done) and thickened epoxy on the bulkheads (which are plywood coated in epoxy). This has resulted in a somewhat uneven surface on the hull for re-attaching the new plywood. Which isn't a big deal because the original plywood didn't follow the curves of the hull that truly anyway and stood off about 1/8" or so on average and the - still attached - resin chunks will act as a spacer to the original positions.

Now I'm looking at reattaching the new stuff which will be 1/8" plywood. I'm thinking of the following alternatives (in my preferred order):
  1. Industrial strength waterproof liquid nails
  2. Flexible tile adhesive
  3. Polyester resin

Opinions and tales of similar experiences welcomed...
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Old 01-09-2015, 09:35   #20
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Re: Glueing plywood to fibreglass.

I would not use liquid nails, as someone said it goes hard and gives way. Likewise tile cement which is not designed to work on a flexing surface. Epoxy is just too much faf and messy. I would go with a gun cartridge 1 part polyurathane. Sikaflex is the one stocked by chandlers and is expensive but works. There are a number of others much cheaper that in my view work just as well, Marinflex in the UK and in the DIY stores Sticks Like Sh*t made by Evo Stik (Bostik) so I guess you should find it easily in the States. It loves moisture (in fact it will not cure without it), is slightly flexible, you can gap fill and if you ever had to, fairly easy to remove. Try some with a pad of ply, after that you will use it for everything, hatches, underwater fittings etc. Its good for DIY also
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Old 01-09-2015, 10:48   #21
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Re: Glueing plywood to fibreglass.

Quote:
Originally Posted by Reefmagnet View Post
A picture is worth a thousand words...

Well, those surfaces don't look to bad. Are you saying you have to replace the bulkheads or just the facing on the bulkheads?
No need to use liquid nails, follow the advice to buy epoxy glue or the other modern materials that come in a caulking tube. They are thick like silicone sealant etc. Not cheap but they are great for what you want to do. Some of them are so strong they are replacing welding in some spots on metal boats!
You could just caulk a vertical heavy strip in maybe 3 places and hold the ply panel inplace until cured.
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Old 01-09-2015, 15:03   #22
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Re: Glueing plywood to fibreglass.

The cheap boat-building guys love PL premium adhesive. I personally haven't tried it but have a tube at home I will use sooner or later. It's been a while since I've done these kind of repairs but back when I did I found this to be the best method:

Take some foam coring (we used to use core cell or something similar) usually about 1/2" think cut them at a 45 degree angle so you have a strip that meets the bulk head then flares to the hull (trapezoid) Bond this to the hull.

Scribe in your new bulk head to sit on the foam pad.

Tab in with polyester.

The pad makes a nice fillet and spreads out the load on the hull. The foam also somewhat adjust to movement in the plywood at least while the polyester is curing.

Epoxy is better but polyester can work well as long as it's prepped properly and you know how to work with it.

I learned about this back in trade school but Gerr mentions it a few times in his books as well.
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Old 01-09-2015, 16:11   #23
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Re: Glueing plywood to fibreglass.

Whilst I understand people's comments re insulation, this may be unnecessary in mid - north Queensland, but would be noticeable in Tassie or similar cooler locations. Probably an extra cost and hassle not warranted in the Banana Bending State though.
For any areas not yet attacked, it may be easier to leave the old ply in place and just attach a veneer over the top. Trimmed to size with a craft knife and you could go mad with the marquetry if feeling so inclined.
I would watch out for some glues, contact adhesive is wonderful but a right whatsit to get the panel aligned on a boat. Use one that lets you move things slightly.
I would agree with the use of spacers first for new areas as it lets the new panel be true to shape and sit flat. Lumps and bumps will occur otherwise because the glue/resin/filler/whatever will shrink as it dries and this tends to pull the board in. Some lumps of it may even partially harden whilst working and the end surface is proud. The battens allow for alignment and you could then just use a Sikaflex adhesive.
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Old 01-09-2015, 16:23   #24
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Re: Glueing plywood to fibreglass.

I replaced the faux teak formica sides of my cabin trunk with 1/8" 1S teak plywood. The surface was a bit rough beneath the formica, but I troweled thickened epoxy on the back side of the plywood with an 1/8" notched trowel. The process is a bit messy but the job was not difficult. The most difficult part involved preparation: cutting, masking, clamping, ect.






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Old 01-09-2015, 18:22   #25
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Re: Glueing plywood to fibreglass.

PL premium polyurethane window and flashing sealant. Not as strong as 5200, but in the same school. Stays flexible.
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Old 01-09-2015, 18:41   #26
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Re: Glueing plywood to fibreglass.

I wouldn't want battens. Unless those dead air spaces are well ventilated, condensate will build up and mildew will follow, in places you can never get into for cleaning.


I'd rather use something like expanding urethane foam, which fills, conforms, and seals and does not absorb water in most products. (You need to check.) It is way strong enough and lasts forever, but can be split with a blade or chisel to remove it if need be.


On the bulkheads...some say to tab them up solid, others say to place a flexible strip between the bulkhead and the hull, then tab across that, so the bulkhead itself can't create a hard spot. I'd suggest looking at West System (Gougeon Bros.) web site at their technical while papers explaining how & why to set the bulkheads.
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Old 01-09-2015, 19:55   #27
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Re: Glueing plywood to fibreglass.

Avoid gaps and dead air spaces by filling with foam, sealing with epoxy. Hidden air spaces are open invitations to infestation.

There are many options on glues. But, so you don't get clobbered by parts of your boat interior, when things get nasty, build it strong enough and avoid cheap shortcuts.
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Old 01-09-2015, 20:22   #28
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Re: Glueing plywood to fibreglass.

Wow heaps of replies. Thanks all. Just to be clear, the ply going back is only cosmetic. Some structural repairs beforehand are being done "by the book", so to speak. The original bulkheads are sound and don't need any structural work.

Putting new ply over the old won't work firstly 'cos all the old ply is gone put for future reference a PO had already taken this course of action in the past and it didn't work. Old man rot and moisture just marched itself from the old to the new. And yes, in the tropics we don't need no steenkin' insulation especially on a boat that has opening windows installed in the hull; The marine world's dumbest idea at least results in a nice cool interior with even as much as a hint of a breeze!

We don't want to totally redesign the interior (well I don't, the missus would go wild left to her own devices ). Basically just want to put it back the way it was hopefully with a glue with some better overall properties than the original resin. I guess I should add, copying the nature of the original installation will involve the glue needing to be troweled on with a tile trowel. Rough as guts, I know, but it worked for 30 years. I'm intending to build in a simple well and drain system to allow for open windows and rain/big heel etc along with some other things to minimize past issues related to water ingress.
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Old 01-09-2015, 20:32   #29
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Re: Glueing plywood to fibreglass.

Fix your leak problems before repairing the interior.
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